ULTRA SONIC PULSE VELOCITY TESTS:

Ultrasonic Pulse Velocity Tests were carried out in accordance with IS: 516(Part-5/Sec 2):2018

In order to determine

• Homogeneity of Concrete
• Development of crack & presence of voids & other imperfection in concrete.
• Changes in the structure of concrete that may occur with time.
• Quality of concrete with respect to standard requirements.

Principles of Ultrasonic Pulse Velocity Tests:

Ultrasonic pulses are generated by an electro-acoustical transducer. It undergoes multiple reflections at the boundaries of the different material phases within the concrete. A complex system of stress waves is developed, which includes longitudinal (compressive), transverse (shear) and surface (Rayleigh) waves. The receiving transducer detects the onset of the longitudinal waves, which is the fastest.

Because the velocity if the pulses is almost independent of the geometry of the material through which they pass, and depends only upon its elastic properties. Pulse Velocity method is a convenient technique for Non-destructive tests for structural concrete.

The underlying principle of accessing the quality of concrete is that comparatively higher velocity is obtained with a good quality of concrete, in terms of density, homogeneity and uniformity. In case of poor quality of concrete, lower velocities are obtained. In case of presence of crack, void or flaw in the concrete, which comes in the way of transmission of the pulses, produces hindrance, thereby making the path length longer. Consequently, lower velocities are obtained. The actual pulse velocity obtained depends primarily upon density and modulus of elasticity of concrete.

APPARATUS FOR ULTRASONIC PULSE VELOCITY TESTS:

• Electrical Pulse generator
• Electro-acoustical transducers – one pair
• Amplifier
• Electronic timing device

METHODOLOGY of Ultrasonic Pulse Velocity Tests

In this test, the ultrasonic pulse is produced by a (transmitting) transducer, which is held in contact with one surface of the concrete member. After traversing a known path length (L) in concrete, the pulse of vibrations is converted into an electrical signal by another (receiving) transducer held in contact with the other surface of the concrete member. An electronic timing circuit enables the transit time (T) of the pulse to be measured. The pulse velocity (V) is given by:

V = L / T

Once the electronic pulse impinges on the surface of the material, the maximum energy is propagated at right angles to the face of the transmitting transducer and best results are, therefore obtained when the receiving transducer is placed exactly opposite, on the opposite face of the concrete member (direct transmission or cross probing). However, in some situations, two opposite faces of the structural members may not be accessible or available for taking readings.  In such cases, the receiving transducer is placed on the same face as of the transmitting transducer (indirect transmission or surface probing). Surface probing is not so efficient as cross probing, because the signal reached at the receiving transducer has amplitude of only 2% to 3% of that in case of cross probing. Also, the results are greatly influenced by the surface layers of concrete, which may have different properties from that of core concrete. The indirect velocity is invariably lower than direct velocity on the same concrete element. This difference may vary from 5% to 20% depending largely upon the quality of concrete.

To ensure that the ultrasonic pulses generated at the transmitting transducer enters the surface of concrete without resistance, and are then ejected out of the surface, into the receiving transducer, it is essential that there be adequate acoustical coupling between the surface of concrete and face of transducer. Typical couplants are petroleum jelly, grease, liquid soap and kaolin glycerol paste. In case of very rough concrete surface, it is required to smoothen and level the area where the transducer is to be placed.

MOUNTING OF TRANSDUCERS:

The direction in which the maximum energy is propagated is normally at right angles to the face of the transmitting transducer, it is also possible to detect pulses which have traveled through the concrete in some other direction. The receiving transducer detects the arrival of component of the pulse which arrives earliest. This is generally the leading edge of the longitudinal vibration. It is possible, therefore, to make measurements of pulse velocity by placing the two transducers in the following manners

DIRECT TRANSMISSION (ON OPPOSITE FACES):

This arrangement is the most preferred arrangement in which transducers are kept directly opposite to each other on opposite faces of the concrete. The transfer of energy between transducers is maximum in this arrangement. The accuracy of velocity determination is governed by the accuracy of the path length measurement. Utmost care should be taken for accurate measurement of the same. The couplant used should be spread as thinly as possible to avoid any end effects resulting from the different velocities of pulse in couplant and concrete.

Remarks: No Correction is applied on the velocities evaluated.

INDIRECT OR SURFACE TRANSMISSION:

Indirect transmission should be used when only one face of the concrete is accessible (when other two arrangements are not possible). It is the least sensitive out of the three arrangements. For a given path length, the receiving transducer get signal of only about 2% or 3% of amplitude that produced by direct transmission. Furthermore, this arrangement gives pulse velocity measurements which are usually influenced by the surface concrete which is often having different composition from that below surface concrete. Therefore, the test results may not be correct representative of whole mass of concrete. The indirect velocity is invariably lower than the direct velocity on the same concrete element. This difference may vary from 5% to 20% depending on the quality of the concrete. Wherever practicable, site measurements should be made to determine this difference.

Remarks: As per IS 516(Part-5/Sec 1):2018 Clause 2.4.3.2.5, Surface probing in general gives lower pulse velocity than in case of cross probing and depending on number of parameters, the difference

could be of the order of about 0.5km/s. In view of this, it is recommended that, in surface probing method the pulse velocity may be increased by 0.5km/s, for values ≥ 3.0km/s.

SEMI-DIRECT TRANSMISSION:

This arrangement is used when it is not possible to have direct transmission (may be due to limited access). It is less sensitive as compared to direct transmission arrangement. There may be some reduction in the accuracy of path length measurement, still it is found to be sufficiently accurate. This arrangement is otherwise similar to direct transmission.

Remarks: Semi Direct velocity is evaluated by addition of 0.5km/s.

CALIBRATION:

The equipment should be calibrated before starting the observation and at the end of test to ensure accuracy of the measurement and performance of the equipment. It is done by measuring transit time on a standard calibration rod supplied along with the equipment.

INTERPRETATIONS:

The quality of concrete in terms of uniformity, incidence or absence of internal flaws, cracks and segregation etc., indicative of the level of workmanship employed can thus be accessed using the guidelines as per the table given below which have been evolved for categorizing of the quality of concrete in structures in terms of the ultrasonic pulse velocity.

Velocity criteria for concrete quality grading

Sr. No.	Pulse Velocity (km/sec)	Concrete Quality grading
1	Above 4.40	Excellent
2	3.75 to 4.40	Good
3	3.0 to 3.75	Doubtful**
4	Below 3.0	Poor

** In case of “Doubtful”, it may not necessary that the concrete is poor, but it may be necessary to carry out further tests like core cutting from structural element and testing the same for compressive strength.

NOTE: In Indirect method 0.5km/s velocity added for values ≥ 3.0 km/s correction as per clause no. 2.4.3.2.5

• FACTORS INFLUENCING ULTRASONIC PULSE VELOCITY TESTS MEASUREMENTS :Influence of Surface Conditions and Moisture Content of Concrete.

Smoothness of contact surface under test affects the measurement of ultrasonic pulse velocity. For most concrete surfaces. When the concrete surface is rough and uneven, it is necessary to smoothen the surface to make the pulse velocity measurement possible.

Pulse velocity through concrete increases with increased moisture content of concrete. This influence is more for low strength concrete than high strength concrete. The pulse velocity of saturated concrete may be up to 5% higher than that of similar dry concrete.

Influence of Path Length, Shape and Size of the Concrete Member.

A concrete is inherently heterogeneous, it is essential that path lengths be sufficiently long so as to avoid any error introduced due to its heterogeneity. If the minimum lateral dimension is less than the wavelength or if the indirect transmission arrangement is used, the mode of  propagation changes and therefore the measured velocity will be different. This is particularly important in cases where concrete elements of significantly different sizes are being compared.

Minimum Specimen Dimensions

1. Influence of Path Length, Shape and Size of the Concrete Member.

Variations of the concrete temperature between 5 ^o C and 30 ^o C do not significantly affect the pulse velocity measurements in concrete. At temperatures between 30 to 60 ^o C there can be reduction in pulse velocity up to 5%. Below freezing temperature, the free water freezes within concrete, resulting in an increase in pulse velocity up to 7.5 %

Influence of Stress.

When concrete is subjected to a stress which is abnormally high for the quality of concrete, the pulse velocity may be reduced due to the development of micro-cracks. This influence is likely to be the greatest when the pulse path is normal to the predominant direction of the planes of such micro cracks. This occurs when the pulse path is perpendicular to the direction of a uniaxial compressive stress in a member.

Effect of Reinforcing Bars.

The pulse velocity measured in reinforced concrete in the vicinity of reinforcing bars is usually higher than in plain concrete of the same composition. This is because the pulse velocity in steel is

1.2 to 1.9 times the velocity in plain concrete and, under certain conditions, the first pulse to arrive at the receiving transducers travels partly in concrete and partly in steel. The apparent increase in pulse velocity depends upon the proximity of the measurements to the reinforcing bar, the diameter and number of the bars and their orientation with respect to the path of propagation. To detect the reinforcing bars, Cover meter test shall be perform. Based on cover meter test data, correction may apply for higher reinforced concrete.

Read more

1. Determination of compressive strength of Concrete Cubes
2. Compacting Factor Test of concrete IS 1199 : 1959
3. Compressive strength of Concrete Core specimen IS 516-1959
4. Slump test of fresh concrete